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Weight limits for Severn Bridge as cables disintegrate

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WORSENING CORROSION on the 40 year old Severn Bridge between England and Wales is to force the Highways Agency to impose weight restrictions on the bridge from this autumn.

Corroded wires in the structure's suspension cables have reduced its load carrying capacity, NCE can exclusively reveal.

Heavy goods vehicles will only be able to travel in one lane in each direction until test results confirm whether more stringent weight restrictions need to be applied.

In some cases the corrosion was found to be so bad that the cables had disintegrated. Water entering the cables during construction or through cracks in the paint work during operation are thought to be the main cause of the damage.

Cables on the Severn have been under examination since April this year, and studies will continue until November (NCE 13 April).

NCE reported in July that cable corrosion was worse than expected, and to a higher degree than experienced on Scotland's Forth Road Bridge (NCE 13 July).

The Forth was the first bridge in the UK where cables were opened up to check for corrosion, after cables in US bridges of a similar age were found to have corroded.

'It's unfortunate that we're not getting any good news from the cable inspections, ' said Highways Agency senior bridge engineer Martin Lynch.

Lynch said that he was certain that the corrosion had caused some degree of strength reduction in the bridge, which warranted the imposition of the weight restriction.

He added that wires were being investigated in two more locations than originally planned and that samples from other locations were currently being load tested. He said that these tests would determine the exact value of the reduction of the bridge's load carrying capacity.

Bridge owner Severn River Crossing has authorised temporary acoustic monitoring next month as a quick measure to check for wire breaks.

The tiny microphones will be located along the cable at low levels where corrosion has already been found.

The permanent system, which will take longer to install, will be operational next year.

Dehumidi ation equipment to dry out the wires and stop further corrosion will be piloted next summer with a full system being installed in 2008.

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